Magnesium and its alloys are characterized by an extremely low density and high strength to weight ratio relative to other structural materials such as steel and aluminum. Thus, magnesium and its alloys have gained increasing acceptance as the structural material of choice for use in industries such as aerospace, automotive, electronics and the like. In its pure state, magnesium is highly reactive. Thus, for most commercial applications, magnesium is alloyed with compatible elements such as aluminum, copper and the like. Alloys of magnesium and aluminum have gained particularly broad acceptance.
Alloys of magnesium may have a relatively high susceptibility to corrosion. This may be particularly true when the alloys are exposed to environments having high salt concentrations such as may exist near seawater. To address this susceptibility to corrosion, it may be desirable to provide coatings across a magnesium alloy part in an attempt to seal the surface from the corrosive environment. One such technique which has been used is electroless nickel coating. While electroless nickel coating provides a hard covering providing a degree of corrosion resistance, the corrosion protection is highly dependent upon the coating porosity. In this regard, due to the highly cathodic nature of the electroless nickel relative to the underlying magnesium alloy substrate, a crack or other flaw in the electroless nickel coating may cause corrosion to be preferentially concentrated at that location. Aside from this deficiency in the corrosion protection mechanism of the electroless nickel coating, it has also been found that such electroless nickel does not provide a suitably stable base for the direct over coating by chromium as may be desired for aesthetic purposes.
It is known to use electroplating to apply protective coatings across a substrate part of aluminum. The ability of an electroplated coating to protect an underlying metal substrate is dependent upon a number of factors. These factors include the position of the metal coating material in the galvanic series, the adhesion between the coating and the underlying layer and the porosity of the coating layer. In order to maintain long-term corrosion resistance, it is generally desirable to promote uniformity of the over-coated layers across the plated part. Such uniformity permits the naturally occurring oxidation and reduction reactions to take place across the entire surface thereby avoiding the possibility of localized corrosive attack.
One commercial electroplating system uses electroplating to apply layers of semi-bright nickel, bright nickel and/or micro-porous nickel across copper coated aluminum parts to provide a multi-layered corrosion resistant system for an aluminum part. The applied coating layers also provide a stable base for adherent over coating by chromium. However, it is not believed that such systems have been used successfully with magnesium or its alloys. In this regard, it is not believed that the layered arrangements used previously with aluminum are suitable to provide the necessary combination of adherence and corrosion resistance if applied to magnesium. Thus, there exists a need for a system for coating magnesium and its alloys which provides both corrosion resistance and a stable base for chrome over plating.